The present invention relates to the determination, by optical emission spectrometry, of the content in a steel of an element such as aluminium, in the dissolved state and in the precipitated state.
The large majority of steels made by continuous casting is steel treated with aluminium. In fact, aluminium confers a certain number of mechanical properties on steel, in particular by its aptitude to form nitrides enabling the size of the grain to be monitored. Furthermore, aluminium is a calming agent.
The aluminium present in the steel is, in particular, in oxidized from (precipitated aluminium, or Al.sub.p) and in solid solution (dissolved aluminium, or Al.sub.d). In order to make steel in optimal manner, it is necessary to know the Al.sub.p content quickly, in order to assess the cleanness of the metal, and the Al.sub.d content in order to know whether the ranges of content envisaged are respected.
Optical emission spectrometry has been currently employed for some years to follow the making of cast iron and steel. Aluminium forms part of the elements metered by this technique.
Optical emission spectrometry consists in subjecting a sample to be analyzed to spark discharging, in directing the optical emission of the arc onto a diffraction grating, and in collecting on a photomultiplier the light beam of the line of which the wave length corresponds to the element to be metered.
According to the method of integration, the current delivered by the photomultiplier is accumulated in a capacitor for a certain time then, when spark discharging is finished, the total charge of the capacitor is measured in order to determine the concentration of the element to be dosed.
It has appeared that the signal delivered by the spectrometer in the course of a spark discharging cycle is formed by components which may be arranged in two groups as a function of their intensities. In fact, if the case of aluminium is considered for example, when, at the point of impact of the spark on the sample, there is an inclusion of precipitated aluminium, the light intensity if much greater than if, at the point of impact, there is only dissolved aluminium. This observation led to imagining the method for so-called sorting (or discrimination) of the pulses. In this latter method, the charge of the integration capacitor is measured, not at the end of a spark discharging cycle, but after each elementary discharge of the excitation circuit and the values measured are stored in order to be subsequently exploited (U.S. Pat. No. 4,326,801).